As the Senate Watergate Committee was turning up the heat on President Richard M. Nixon and his closest associates in 1973, chairman Sam Ervin, a North Carolina Democrat, became something of a national folk hero. At his death in 1985, one newspaper remembered him as “a latter-day Diogenes bent on finding the truth in an era of Watergate lies.” But to Nixon, the courtly, 76-year-old Ervin was something else altogether. “The president of the United States can’t be kicked around …